The present invention relates to storage systems, and in particular, to a method and apparatus for storing data on a virtual tape storage system.
A virtual tape storage system is a hardware and software product configured to interact with a host computer. Application programs running on the host computer store data output on tape volumes for storage. These tape volumes are embodied in the virtual tape storage system as virtual volumes on virtual tape drives (VTD). A virtual volume is a collection of data, organized to appear as a normal tape volume, residing in the virtual tape storage system. To the host computer and to the application programs, the tape volume contents appear to be stored on a physical tape device of a particular model, with the properties and behavior of that model emulated by the actions of the virtual tape storage system. However, the data may actually be stored as a virtual volume on any of a variety of different storage mediums such as disk, tape, or other non-volatile storage media, or combinations of the above. The virtual volume may be spread out over multiple locations, and copies or "images" of the virtual volume may be stored on more than one kind of physical device, e.g., on tape and on disk.
When an image of the virtual volume is stored on disk, different portions of the volume's contents may be stored on different disk drives and on different, non-contiguous areas of each of the disk drives. The virtual tape storage system maintains indexes which allow the contents of any virtual volume whose image is stored on disk to be read by the host, the virtual tape storage system retrieving scattered parts as needed to return them in correct sequence.
When an image of a virtual volume is stored on tape, it may be stored on a single tape together with images of other virtual volumes, or different parts of the image may be stored on more than one different tape with each part again placed with images, or parts of images, of other virtual volumes. In both of these approaches to tape storage of virtual volume images, the images are said to be "stacked." The virtual volume images may be stored on a variety of different tape device models other than the one being emulated. As with images stored on disk, the virtual tape storage system maintains indexes which allow it to retrieve the contents of any virtual volume stored in a stacked image from the tape or tapes on which it is stored.
A shortcoming of storing stacked images on tape arises because the stacked image is not recognizable by standard hardware and application programs.
Existing virtual storage systems include proprietary tape drive units for destaging virtual volumes from staging disks to tape. If, as is usually the case, the customer has already invested in tape library hardware the addition of a virtual tape drive system requires adding additional tape drive resources to perform destaging operations for the virtual tape drive system.
Thus, an improved virtual tape system and methods for its operation that overcomes the shortcomings of the presently available devices is needed.